Engaging in regular physical activity (PA) is consistently inversely associated with risk of breast and colon cancers, 3-17 with additional evidence for suggesting PA helps prevent endometrial,18,19 pancreatic,20 and lung21,22 cancers. Despite these benefits, less than 5% of overweight and obese adults meet national guidelines for PA.44 Moreover, overweight adults spend less time in PA than normal weight adults,44-48 and are more likely to discontinue PA programs.49,50 Affect (feeling good/bad) is important for promoting regular PA, as those who respond with less positive affect during moderate intensity PA are less likely to adopt a program of regular PA.23-28 Additionally, those experiencing lower positive affect and higher negative affect outside the context of PA are less likely to engage in PA.29-34 Given the importance of positive affect for adoption and maintenance of regular PA, intervention componenents that focus on enhancing positive affect may be a valuable addition to standard PA promotion interventions. Specifically, interventions derived from positive psychology55 have shown promise in reducing depressive symptoms and increasing positive affect and are beginning to be applied to other areas of behavioral health.56-57 Consistent with the ORBIT model52 for developing behavioral interventions, the aims of this project are to, first, translate positive psychology theory into a 6-week, group-based positive psychology for PA promotion (PPPA) intervention for low-active overweight or obese adults (>18 years; BMI 25-40), delivered at local YMCAs, and supplemented with text messaging. We will deliver the intervention prototype among a cohort of 10 participants. Participants and investigators will provide ongoing feedback regarding their experiences with the intervention in an iterative development process to refine the protocol. Our second aim is to test proof-of-concept and feasibility of PPPA in the context of a randomized pilot study among 60 low-active overweight or obese adults at local YMCAs. In an additive design, participants will be randomized in a 2:1 ratio to PPPA versus a control intervention including only the standard PA promotion components of the PPPA intervention (i.e., PA education, self- monitoring, and goal-setting), with equal frequency of staff contact and text message delivery. All participants will be followed for 6 months, and will receive a 6-month YMCA membership to equate access to PA facilities. We hypothesize that participants in PPPA, relative to the standard PA intervention will: demonstrate equal or greater session treatment retention and satisfaction (H1); and more min per week of PA as measured by accelerometry at immediate post-treatment (week 7) and weeks 13 and week 26 (H2). As a secondary aim we will examine effect sizes for PPPA versus the standard PA intervention on putative mediators that may underlie the efficacy of PPPA in improving PA outcomes, including positive and negative affect, optimism, happiness, life satisfaction, social support for PA, and PA enjoyment. The proposed research will set the stage for an RCT to test a novel PA promotion intervention that can be readily disseminated.